Interaction of doxantrazole with calcium ions in solvents of intermediate polarity. Possible effect of ionization of the drug.

Spectral evidence is presented that 0.01-0.03 mM solutions of the antiasthmatic drug doxantrazole contain the neutral or ionized form, depending on the solvent and, at least in 1 case, on concentration. In 1,4-dioxane, where only the neutral form exists, and in water, where the drug is ionized, no reaction between doxantrazole and calcium ion can be demonstrated spectroscopically. However, in 2-propanol at 0.01 mM and in 7:3 (v/v) 1,4-dioxane-water the doxantrazole anion can be shown to react with calcium. In more concentrated 2-propanol solutions most of the drug is in the neutral form, and no reaction with calcium is evident. It is suggested that in a biological system doxantrazole can react directly with calcium but such a reaction will depend on the state of the drug and the nature of its environment.